1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD), and more particularly, to a multi-domain liquid crystal display device that could have simplified manufacturing processes, and can provide a wide viewing angle.
2. Description of the Related Art
Generally, a liquid crystal alignment layer of a TN (Twisted Nematic)-LCD that is used as a LCD having a wide-area and high image quality is provided inside of a transparent electrode, and which is adjacent to liquid crystal molecules. In the interface of an alignment layer and liquid crystal molecules, alignment of liquid crystal molecules can be expressed with a uniaxial alignment property from a uniaxial extension method of an alignment layer, and an angle (pre-tilt angle) between the alignment layer and the liquid crystal molecules.
The conventional rubbing method has been widely used as a means for applying an uniaxial extension process on a liquid crystal alignment layer so as to obtain a wide area and a high speed process and to simplify the manufacturing processes. Rubbing a substrate coated with polymer with a cloth is a simple method.
If a substrate were rubbed by the method, micro grooves should be formed on the alignment layer. These micro grooves are aligned parallel with liquid crystal molecules so as to minimize the elastic deformation energy. But in the rubbing method, shapes of micro grooves formed on the alignment layer depend on the friction intensity (rubbing strength) of the rubbing cloth and the alignment layer so that alignment of liquid crystal molecules is non-uniform, and phase distortion and light scattering problems are generated. The problems exert an important defect to the performance of the LCD.
Therefore, in this rubbing method, the pre-tilt angle changes precisely according to the application condition or the rubbing condition so that a realization must be considered sufficiently. Further, the polymer layer is rubbed with the rubbing cloth so that a minute dust or an ESD (Electrostatic discharge) may be generated. The dust will be a large obstacle in a process of forming a TFT (Thin Film Transistor) that is provided by repeat of a high-minuteness pixel electrode, application, exposure and etching processes. A local discharge will result in damage of the alignment layer itself, the disconnection of a transparent electrode or TFT, or static electricity destruction. And an alignment layer for a TFT must have a high voltage holding ratio that is able to maintain a charge for a long time.
Particularly, the light transmission of a TN-LCD has a symmetric distribution versus viewing angle in the right-left (horizontal) direction, but has an asymmetric distribution versus viewing angle in the up-down (vertical) direction. In the viewing angle along the vertical direction, an image inversion range is generated and the viewing angle becomes narrower.
To supplement the anisotropy in the liquid crystal, a multi-domain TN-LCD as a TDTN (Two Domain TN)-LCD and a DDTN (Domain Divided TN)-LCD has been introduced. A photolithography and a rubbing method can characterize a method of manufacturing this multi-domain TN-LCD. That is, photolithography and rubbing methods must be executed at least twice so as to form a domain that has mutually opposite or different alignment directions within each pixel. And a four-domain TN-LCD can be also obtained as a more improved structure.
But a two-domain TN-LCD has an up-down (vertical) viewing angle of only ±25° for the contrast ratio to be beyond 10, and the up-down (vertical) viewing angle of a four-domain TN-LCD is also about ±40°. Further manufacturing processes are complex, and the stability of a liquid crystal cell itself or instability problems of the tilt angle remain as before.
In order to overcome this problem in the rubbing method, a photo-alignment method that determines an alignment direction by two exposures to ultraviolet rays irradiation has been introduced.
However this photo-alignment method has problem of weak anisotropy in the photo-alignment layer and weak anchoring energy problem by that, or a non-uniform alignment of a liquid crystal, a disclination by interacting with a liquid crystal, and flow spots generated when a liquid crystal is injected. Also, irradiation four times is necessary to form one domain by applying irradiation twice on each of the upper and lower substrates. Therefore, in order to form a multi-domain structure, the irradiation numbers progressively increase for one pixel, so that irradiation processes progressively become complicated.